Removable prosthetic dentures typically consist of a baseplate of hard poly(methyl methacrylate) which supports the artificial teeth needed for chewing and for esthetics. Many patients have difficulty tolerating a hard denture so there is a need for a soft liner affixed to the denture base to cushion the soft tissue. A number of criteria have been established for materials to be useful as denture liners, notably non-toxicity and non-allergenicity. The best materials should be permanently resilient, inert, cleanable, substantially water-insoluble, have low water sorption characteristics and good tensile and bond strengths. While softness is desirable for comfort, the liner material must be sufficiently firm to displace the soft tissues of the mouth and to allow grinding of the denture periphery to avoid creating sore spots on the tissues; additionally, the liner material must be permanently bondable to the denture base material. It is also preferable from a practical standpoint that the lined denture be capable of fabrication under conditions generally found in dental laboratories or in a dentist's office, avoiding extremes of temperature and pressure conditions, or the use of special equipment.
Several liner materials have been proposed which satisfy these criteria sufficiently to be useful, such as silicone rubbers, plasticized poly(methacrylates), polyurethanes and polyvinyl chlorides. An especially useful liner material is disclosed in U.S. Pat. No. 4,251,215 issued on Feb. 17, 1981 to May et al, comprising phosphonitrilic fluoroelastomer (poly(fluoroalkoxy)phosphazene) which exhibits particularly good resiliency and biocompatibility characteristics. The fluoroelastomer liner materials exemplified therein, however, have been found to be somewhat deficient with respect to tensile strength, hardness, water-sorption and bondability to denture base material, as compared to the theoretical ideal. Further, the process for forming the denture as described in the May et al patent is a two-step process which requires the liner and denture base to be separately cured, and additionally requires the use of strong bonding agents, as well as the use of temperatures in excess of 100.degree. C. (boiling point of water at atmospheric pressure) for curing the liner. The fabrication of this prior art composite denture thus requires somewhat elaborate equipment, and cannot be conveniently accomplished in an average dental office or laboratory.
In order to overcome the drawbacks of such prior art compositions, liner compositions of the type described in U.S. Pat. No. 4,432,730 to Gettleman et al, filed Oct. 1, 1982 and incorporated herein by reference, have been proposed. While such dental liner compositions have proved generally satisfactory, water sorption tendencies of the cured liner material according to the Gettleman et al patent have been higher (4-5% w/w) than desirable for optimum use in the mouth. This composition also used methyl methacrylate (MMA) monomer, which is volatile and therefore difficult to control prior to curing; hardness and water sorption properties of these prior art compositions have been found to be highly dependent upon the proper amount of MMA. It is thus desirable to decrease water sorption characteristics and concomitant swelling of this prior art dental liner material, in order to improve dimensional stability of the product liner material over time.